Tubular solid oxide fuel cells (SOFCs) represent a significant advantage over planar-type SOFCs due to enhanced gas collection capability, ease of manufacture, and strength of the tubular design. Anode supported tubular SOFCs possess additional advantages over cathode or electrolyte supported cells due to lower cost, greater strength, and more intimate relationship with the critical gas component, i.e., the fuel. With this capture of the fuel, they also inherently have the ability to perform on-cell reformation of fuels rather than require external reforming equipment.
FIG. 1 is a cross-sectional view of a typical anode-supported tubular SOFC as known in the art. Generally speaking, an anode-supported tubular SOFC has a hollow, tubular inner anode layer 102, an electrolyte layer 104 formed on a portion of the outside of the anode layer 102, and a cathode layer 106 formed on a portion of the electrolyte layer. Current flows radially from the inside to the outside along the length of the tube.
As shown in FIG. 2, current collection in anode-supported tubular SOFCs typically involves anodic electrical take-off connections 202 and cathodic electrical take-off connections 206 located at one end of the tubular fuel cell adjacent a fuel input 220. This arrangement allows mechanical ease of assembly, utilizing the gas distribution manifolds as current collection devices. For current collection, wires must be run between the manifold and the cathode and/or anode. Due to separation and connection constraints, the manifold must be designed to allow for sufficient spacing to accommodate these connections, resulting in a relatively large system. Additionally, this arrangement generally results in large electrical power losses, proportional to the length and thickness of the anode supported fuel cell.
One drawback of the current collection arrangement shown in FIG. 2 is that the current needs to travel along the entire length of the tube. This can result in major power losses. FIG. 3 shows a representation of current decreasing as a function of increasing tube length for an anode-supported tubular SOFC having anodic and cathodic current collectors at one end of the fuel cell. It is therefore desirable to reduce or minimize these losses to enhance cell performance and lower fuel cell costs.
Siemens Westinghouse describes the use of a single strip down the length of a cathode supported fuel cell, allowing current collection along the length, with only circumferential losses, although due to the design of their cathode-supported fuel cell, significant non-uniform circumferential stresses can be formed. With such a design, improved current collection is generally realized at the expense of a more complicated system design and greater variability in the packing of the tubular fuel cells.